Field of the Invention
Exemplary embodiments of the present invention relates to a light emitting diode (LED), and more particularly, to a GaN-based high-efficiency LED in which a growth substrate is removed using a substrate separation process.
Discussion of the Background
In general, since Group-III-element nitrides, such as gallium nitride (GaN) and aluminum nitride (AlN), have an excellent thermal stability and a direct-transition-type energy band structure, they have recently come into the spotlight as materials for light emitting diodes (LEDs) in visible and ultraviolet regions. Particularly, blue and green light emitting devices using indium gallium nitride (InGaN) have been used in various applications such as large-sized full-color flat panel displays, traffic lights, indoor illuminators, high-density light sources, high-resolution output systems and optical communications.
Since it may be difficult to form a homogeneous substrate on which a Group-III-element nitride semiconductor layer can be grown, the Group-III-element nitride layer may be grown on a heterogeneous substrate having a crystalline structure similar to the nitride semiconductor layer through a process such as metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). A sapphire substrate with a hexagonal system structure may be used as the heterogeneous substrate. However, since sapphire is an electrical non-conductor, the structure of an LED using a sapphire substrate may be limited. Accordingly, there has recently been developed a technique in which epitaxial layers such as nitride semiconductor layers are grown on a heterogeneous substrate such as a sapphire substrate, a support substrate is bonded to the epitaxial layers, and the heterogeneous substrate is then separated using a laser lift-off technique or the like, thereby fabricating a high efficiency vertical LED (for example, see U.S. Pat. No. 6,744,071, issued to Sano, et al.).
Generally, a vertical LED may have an excellent current spreading performance thanks to a structure in which a p-type semiconductor layer is positioned at a lower portion in the vertical LED as compared with a conventional horizontal LED, and may have an excellent heat dissipation performance by employing a support substrate having a thermal conductivity higher than that of the sapphire substrate. Further, light emitted toward the support substrate can be reflected by disposing a reflective metal layer between the support substrate and a p-type semiconductor layer, and a roughened surface may be formed on an n-type semiconductor layer by anisotropically etching an N-face through photo-enhanced chemical (PEC) etching or the like, so that the upward light extraction efficiency may be considerably improved.
However, since the entire thickness (about 4 μm) of an epitaxial layer may be very thin as compared with the light emitting area, for example, of 350 μm×350 μm or 1 mm2, it may be very difficult to implement the current spreading. To solve such a problem, a technique for promoting current spreading in an n-type layer involves using an electrode extension extending from an n-type electrode pad, or current may be prevented from directly flowing from the n-type electrode pad to a p-type electrode by disposing an insulating material at the position of the p-type electrode corresponding to the n-type electrode pad. However, there may be a limitation on preventing the current flow from being concentrated from the n-type electrode pad toward a portion just under the n-type electrode pad. Moreover, there may be a limitation on uniformly spreading the current throughout a wide light emitting area.
Particularly, the current concentration may accumulate fatigues in a partial region of the LED, i.e., a region on which the current is concentrated, and therefore, a leakage current path may be formed in the region. For this reason, the current concentration in the region just under an electrode pad may hinder applying the LED having the vertical structure as an LED for illumination, which requires a high reliability. Particularly, in case of a high-luminance LED used for illumination, a minute current concentration may deteriorate the light emitting efficiency of the LED and may have a bad influence on the lifespan thereof.
Meanwhile, a process for fabricating a vertical LED, e.g., a process of growing an epitaxial layer on a growth substrate or bonding a support substrate to the epitaxial layer, is performed at a relatively high temperature. The growth substrate, the epitaxial layer and the support substrate may have different thermal expansion coefficients from one another. Hence, after the process is completed at the high temperature, a stress is applied into the relatively thin epitaxial layer, thereby inducing a residual stress. While the growth substrate may be separated through a laser lift-off process, physical damage such as cracks may be easily generated in the epitaxial layer by the residual stress. Moreover, a shock wave may be transferred to the epitaxial layer due to the emission of a laser beam in the laser lift-off process, and therefore, which may damage the epitaxial layer.
In addition, a surface of the epitaxial layer may not be flat and may have a locally concave or convex portion due to the difference of thermal expansion coefficients between the growth substrate and the epitaxial layer. Accordingly, when the support substrate is bonded to the epitaxial layer, micro-bubbles may be formed between the epitaxial layer and the support substrate.